How is your schedule? Calculating the real SPI indicator for your project.

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Most project managers know the SPI indicator – which should be indicating your schedule performance. Most project managers also probably recognize that once the project is already late, the SPI does not correctly reflect the real status of the project, and worse – the schedule may be slipping, but the SPI is improving…

There is a great presentation by Kym Henderson and Walt Lipke available here discussing a new way of calculating your SPI – Earned Schedule. I really recommend going over this presentation to really understand the heart of the matter, but in short the bottom line is that instead of calculating the SPI in the normal way (BCWP/BCWS) which they refer to as SPI($) or the cost based SPI, a new SPI indicator should be used – the SPI(t) – or the time based SPI indicator.

The logic behind ES (Earned Schedule) is that you know where you are compared to where you should have been – a key piece of data that may sound trivial, but isn’t at all. In the presentation there are visual ways to calculate and get to the ES value, but how do you do it automatically?

I was confronted with this question while developing and internal project management application, and will share my solution here. (This is where we get a little technical)

In order to calculate the ES, we need to know two key data points:

How much work have we done until now?

To which point in time in the project schedule does this amount of work belong?

#1 is obvious and easy to get to. But how do we get #2?

The solution is going over the entire schedule and calculating the cumulative amount of work for every day in the project (if days are your work breakdown resolution). Now that we have a very long list with the amount of work that should have been completed for every day in the project, we can compare the number we got in #1 and when we found our day in the list, we know where we are in the schedule.

Now we only need to compare this date with the current date to know if we’re on schedule, behind or ahead. The great thing about ES is that now we have a REAL picture of the schedule – and we can actually use the same method as is used with CPI – as you can estimate your cost at completion, by with multiplying the SPI(t) value we got to the project length you can get the estimated project length.